1. Field of the Invention
The present invention relates to a converter and a driving method thereof. Particularly, it relates to an interleaved converter formed by connecting two converters in parallel and having one input terminal and one output terminal by respectively connecting input terminals and output terminals of the two converters, and a driving method of the interleaved converter.
2. Description of the Related Art
FIG. 1 shows a conventional converter. The conventional converter includes an inductor 900, a switch 901, a diode 902, and a capacitor 903. An inductor current iL flows through the inductor 900 according to an input voltage Vin and an output power, and charges the capacitor 903. A voltage charged into the capacitor 903 becomes an output voltage of the converter. In the interval when the switch 901 is in the turn-on state and the diode 902 is in the turn-off state, the inductor current iL increases as time passes, and the inductor 900 stores energy. In the interval when the switch is turned off and the diode is turned on, the inductor current iL charges the capacitor 903 through the diode 902. In this interval, the energy stored in the inductor 900 is transmitted to the capacitor 903, and the inductor current iL decreases as time passes.
A driving mode of a converter can be classified into a continuous conduction mode (CCM), a discontinuous conduction mode (DCM), and a boundary conduction mode (BCM) according to current waveforms flowing through an inductor. In the CCM, the inductor current of the converter continuously flows for one switching period. In the DCM, there is an interval during which the inductor current of the converter does not flow within one switching period. In the BCM, the switch is turned on at the time when the inductor current becomes zero. In the CCM, a reverse recovery current flows when the diode is turned off. In the DCM and BCM, the switch is turned on when the inductor current is zero, and thus no reverse recovery current is generated. Accordingly, a diode loss energy in the DCM and BCM is lower than that in the CCM.
In general, BCM is preferred than DCM since the non-conducting period of the inductor current cause line current distortion. In BCM operation, the switch is turned on at the moment when the inductor current reaches zero. If the switch conduction time is fixed, the peak current becomes proportional to the input voltage. Since the average of inductor current is half of the peak value, the average of inductor current naturally follows the shape of line voltage and the sinusoidal input current is obtained. The inductor size can be also reduced with BCM operation since the inductance value can be much smaller than that of CCM operation.
Even though the BCM operation has a lot of advantages such as simple control and less reverse recovery loss, the high RMS current causes high conduction losses in the inductor and switching device. The line filter size also should be much bigger than that of CCM PFC converter due to high ripple current. Therefore, BCM PFC converter has been applied relatively low power application below 200W. Considering the overall system size and power conversion efficiency, using BCM PFC for power level above 200W has practically has no merit. In order to overcome this drawback and apply BCM for higher power level, in some designs, two converters operated in the BCM are controlled to have a phase difference of 180 degrees. In this design the ripples flowing through the inductors of the two converters cancel each other so that the current ripple at the input terminal can be reduced, as in the CCM. Such a driving method is called interleaved driving.
In the interleaved driving, a master-slave method is most generally used. A master converter independently operates at the BCM, and a control circuit of the master converter detects a time when an inductor current becomes zero by observing an inductor voltage and turns on a switch (hereinafter referred to as a master switch) of the master converter. A control circuit of the slave converter calculates a center point of one switching operation period of the master converter, and turns on a switch (hereinafter referred to as a slave switch) of the slave converter. In this instance, both of the two converters can be operated in the BCM by controlling the turn-on times of the master and slave switches.
However, the master-slave designs can have a problem in that the center point of any specific switching operation period of the master switch can be detected only after the specific switching operation period is fully observed. In other words, the center point of the specific switching operation period of the master switch cannot be detected until the specific switching operation period of the master switch is fully observed. Another problem is that a switching operation period of the master switch operated at the BCM continuously varies according to the operation conditions of the converter, including the input voltage and output power, and therefore it is difficult to precisely detect a center point of the one switching operation period of the master switch.
This problem can be solved e.g. by using a previous switching operation period of the master switch since the switching frequency changes slowly according to the 60 Hz line frequency and the previous switching operation period is typically similar to the one switching operation period. Therefore, a converter using a conventional interleaved method remembers a previous switching operation period by using a sample and hold circuit and uses the previous switching operation period for determining a center point of the next switching operation period. The conventional interleaved converter has, however, a disadvantageously complicated circuit structure since it uses the sample and hold circuit to retain the information regarding the previous switching cycle.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.